1. Field of the Invention
The present invention relates to a toner used in an image forming method such as an electrophotography method, an electrostatic recording method and the like, and particularly to a toner suited for heat roller fixing.
2. Related Background Art
A large number of processes have been conventionally known as electrophotographic processes, as disclosed in U.S. Pat. No. 2,297,691, Japanese Patent Publication No. 23910/1967 (U.S. Pat. No. 3,666,363) and Japanese Patent Publication No. 24748/1968 (U.S. Pat. No. 4,071,361). In general, copies are obtained by forming an electric latent image on a photosensitive member utilizing a photoconductive material and according to various means, subsequently developing said latent image by use of a toner to form it into a visible image, and transferring the toner image to a transfer medium such as paper as necessary, followed by fixing by heat or pressure, or heating and pressurization.
Also known are various types of developing processes in which an electrostatic latent image is formed into a visible image by use of a toner. For example, there are such processes as the magnetic brush development disclosed in U.S. Pat. No. 2,874,063, the cascade development disclosed in U.S. Pat. No. 2,618,552, the powder cloud development disclosed in U.S. Pat. No. 2,221,776, the fur brush development and the liquid development.
Also known is a magnetic recording process in which a magnetic latent image is developed by a magnetic toner.
As toners used in these development processes, there has been conventionally used fine powder obtained by dispersing a dye or pigment in a natural or synthetic resin. It is also known to use developing fine powder to which a third component is added for various purposes.
The developed toner image is transferred to a transfer medium such as paper and fixed thereto, as necessary. Known methods of fixing the toner image include a method in which a toner is melt-adhered and hardened on a support by heating and pressurizing the toner by a heat roller, a method in which a toner is heat-melted by use of a heater or a heat roller so as to be melt-adhered and hardened on a support, a method in which a binder resin in a toner is softened or melted by use of an organic solvent and then fixed on a support, and a method in which toner is pressure-fixed on a support.
Materials for toners are selected so as to be suited for the respectively corresponding fixing methods and, in general, a toner used in a particular fixing method can not be used in the other fixing methods. In particular, it is almost impossible to divert a toner used i the widely available heat-melt fixing method employing a heater, to the heat-roller fixing method, the solvent fixing method or the pressure fixing method. Accordingly, there have been studied and developed toners correspondingly suited for the respective fixing methods.
Various methods and apparatus have been developed with respect to the steps of fixing a toner image to paper, and the methods most widely used at present may include the heat-roll fixing method in which heat and pressure is simultaneously applied. The heat-roll fixing method is a method in which an image-receiving sheet supporting a toner image is brought into contact with a heated roller, thereby fixing the toner image on the image-receiving sheet.
However, this fixing method tends to generate an undesirable offset phenomenon in which part of the toner supported on the image-receiving sheet is transferred on the surface of the roller.
As disclosed in Japanese Patent Publication No. 23354/1976 (U.S. Pat. No. 3,941,898), such an offset phenomenon tends to be caused when a low molecular resin is used as a binder resin of a toner. For this reason, as disclosed in this publication, a crosslinked resin has been used as a binder resin, whereby the offset phenomenon is considered to be able to be prevented to a certain extent. As a matter of course, mere employment of the crosslinked resin may result in the increase of the fixing temperature, causing the problem of the low-temperature offset at an unfixed portion, without reaching the level of practical use.
Further, a toner is required also to have, besides the fixing performances, excellent blocking resistance, developing performances, transfer performances, cleaning performances in an appropriate proportion.
Conventionally, the toner used in the heat-roller fixing method has been usually produced by employing a process in which a thermoplastic resin, a coloring agent, a release agent such as polyalkylene, and other additives are melt-mixed and thereafter, after cooling the resulting kneaded material, the cooled material is ground by a finely grinding machine, wherein the ground material is controlled to have a desired particle size distribution by a classifying machine. This production process can produce a considerably superior toner, but may have a certain limit.
In this production process, which requires a grinding step, the cooled kneaded material must be ground at a considerable speed to attain the desired particle size taking account of the productivity and depending on the type of a grinding apparatus. For this reason, it is necessary to use a material having a brittleness high enough to be readily ground by a mechanical force. However, a toner having a high brittleness may be formed into fine particles because it may be readily ground by a load applied to the toner in a developing device, such that it may contaminate a developing sleeve, or may make imperfect by itself the charge control, bringing about undesirable phenomena such as fogging.
When a low melting material is used in order to improve the heat stability of a toner, the resin composition may be melt-adhered to the grinding apparatus, resulting in an extreme lowering of the productivity.
Further, the toner produced by such a production process has disadvantages in that the surface thereof comprises part covered with resin and part on which a pigment is exposed to have an non-uniform surface property, and therefore the distribution of the frictional charge quantity of a toner can not be uniform, or, when a toner is produced by using a low softening or low melting resin as a binder resin in order to improve the fixing performances, the flowability of a toner may be so worsened that the toner can not be applied in practical use. Still further, in the toner obtained by the melt-kneading method, the release agent such as low molecular weight polyalkylene has such poor compatibility with a binder resin in nature that the dispersing state of the release agent can be controlled only with difficulty. The dispersing state of the release agent may vary depending on the temperature, time and share in melt-kneading, and may greatly affect the fixing performances, blocking resistance of a toner. The reason thereof is presumably that the amount of the release agent added in each of the ground toner is different, or that the particle size of the release agent in a toner can be greatly varied. In order for these release agents to be immediately diffused to the surface of a toner at the time of fixing to achieve the release effect, they are essentially required to have poor compatibility with a binder, and it is difficult to obtain a toner having desirable dispersibility by a melt-kneaded material grinding method.
To solve the above various problems, there has been proposed, as disclosed in Japanese Unexamined Patent Publication No. 87051/1981 (U.S. Pat. No. 4,514,487), a process for producing a toner, characterized by carrying out polymerization in the presence of a release agent. According to this process, the release agent present in a monomer during the polymerization step is melted by heat to turn to a liquid, whereby the uniform distribution into the monomer can be surely achieved. However, because of the change in the solubility of the release agent due to the change of the polymerization degree during the polymerization, the state of precipitation may vary with time, and the control of particle size of the release agent can be made with difficulty as in the kneading method, whereupon great non-uniformity can be seen in the fixing performances.